This invention generally relates to semiconductor laser arrays and more particularly, to a semiconductor laser array and a method of forming the laser array where the laser beams emitted are directed perpendicular to or parallel along the surface of the chip by the use of a high index material.
Semiconductor laser arrays and laser lines have many potential applications including high power sources to pump solid state lasers and a variety of optoelectronic applications. Much of the current effort in laser arrays is devoted to linear arrays of edge-emitting GaAs/AlGaAs lasers, fabricated by growing AlGaAs alloy layers on a GaAs substrate and cleaving the GaAs substrate at both ends of the lasers to form the two laser mirrors of the resonant cavity. Several lasers or a row of lasers may be formed together by this process. Limited success has been experienced in fabricating two-dimensional arrays of lasers by stacking and joining several discrete linear laser arrays formed by such cleaving techniques together.
Other efforts include building monolithic two-dimensional laser arrays which have been fabricated as a grid of semiconductor devices which emit laser beams perpendicular to the surface of the chip. Prior art devices in this category include arrays which use lasers with their resonant cavities normal to the wafer surface, arrays with chemically etched 45.degree. mirrors or arrays utilizing a second order grating coupler. However, the efficiencies of most of these prior art lasers were too low to be of practical interest. A more promising technique is to use devices formed by coupling edge emitting laser diode with external mirrors formed by etching 45.degree. parabolic or slant mirrored surfaces on the chip. This technique is described in an article by T. H. Windorn and W. D. Goodhue, entitled "Monolithic GaAs/AlGaAs Diode Laser/Deflector Devices for Light Emission Normal to the Surface," Appl. Phys. Lett. 48 (24), 16 June 1986.
There are, however, several problems with the prior art which limit the power, size and manufacturability of two-dimensional laser arrays.
Forming the mirrors of the resonant cavity by cleaving is suitable for the fabrication of a linear array of lasers. However, in a linear array, the narrow shape of the substrate formed by cleaving sharply limits the location and amount of support circuitry and wiring that can be integrated in the laser substrate. Support circuitry such as laser control logic, memory devices, decoders, input-output ports and power supplies can only be accommodated on the two remaining sides of the laser. Whether these support devices are most ideally placed adjacent to the laser or at the periphery of the substrate, the cleaved mirror technique presents severe obstacles to the chip designer. Another problem with the cleaving technique is that electrical testing of the devices cannot be performed until the laser chips are mounted in their packages.
While limited success has been experienced in fabricating two dimensional arrays from a series of cleaved linear laser arrays, these arrays share all of the same problems of the linear arrays and add some new ones of their own. Discrete wires are used to interconnect the stacked laser arrays. The numerous external electrical connections raise considerable reliability concerns when compared to the interconnecting metallurgy used on an integrated chip. In addition, interference among the laser beams determines the light pattern of the laser array, and as the interference pattern is in part determined by the position of the lasers, the positioning of the lasers within the array must be precise. It is quite difficult to form an array of stacked lasers, consequently the positioning of the laser elements within the array can vary significantly from array to array.
A monolithic two dimensional laser array avoids some of the problems inherent in cleaving the substrate to form laser mirrors allowing greater freedom in locating support devices and greater precision in the positioning of laser elements in the array. However, the 45.degree. parabolic or slant mirrors used in the prior art are formed by depositing a reflecting metal film on the slanting surface on the substrate. Mirror reflection is not 100 percent efficient and particularly at some wave lengths of light, significant power is lost at the mirrored surface. As a result, in addition to losing light output, the heat which must be dissipated by the chip is higher. One of the major problems limiting the size of the laser array is that it becomes progressively more difficult to remove waste heat as the size of the array increases. It would be clearly advantageous to eliminate the power loss at the mirror surface. Other problems in using metallic mirror surfaces include potential corrosion concerns, and mechanical problems including difficulties of attaching optical fibers to mirror reflectors mounted at a sloping angle.
Topology concerns play a significant factor in fabricating large laser arrays. Using current deposition methods, the laser can be significantly higher than the substrate on which it is situated. The side edges of the lasers present sharp topographical features to the overlying interconnecting metallurgy. To reduce yield and reliability problems, a gradually sloping spacer can be included at the side edges of the laser. However, these sloping spacers use up chip area, reducing laser and circuit packing density. All of these factors pose problems in achieving functional and reliable metalization connecting support circuit elements on the supporting substrate adjacent to the laser.